This invention relates to a nozzle which, in its typical use, is submerged in molten metal during steel casting and which is used in guiding molten metal from a tundish to a mold, especially in a continuous steel casting apparatus.
In a conventional steel casting apparatus which uses a submerged nozzle, argon gas is blown into molten metal which is moving down through the submerged nozzle. The gas is introduced through the nozzle in order to avoid the deposition of unwanted steel onto an inner nozzle surface, resulting in a blockage.
The argon gas moves, along with the molten metal flow, into and out of the submerged nozzle, and then floats to the surface of a molten metal in a mold where a mold powder layer exists. When this happens, the gas moves from molten steel, which has a higher specific gravity, to a mold powder layer having a lower specific gravity. At the boundary surface, therefore, the volume of the argon gas suddenly expands and the gas bubbles burst.
This bursting, accompanied by the drastic change in gas volume, agitates the mold powder layer so that the molten metal damages a part of the nozzle ("nozzle powder line section") that is in contact with the mold powder layer.
In a steel casting apparatus which incorporates a submerged nozzle, the demand for multiple continuous casting and enhanced service life has recently increased, reflecting a need to obtain operating advantages and to reduce production cost.
In general, since wear of the nozzle powder line section presents a most critical problem in terms of service life, a ZrO.sub.2 -C material displaying excellent anti-corrosion properties has been used for the powder line section of submerged nozzles.
In an attempt further to improve the wear-resistance of the powder line section of such nozzles, the thickness of the powder line section was increased, in hopes of prolonging the service life of the powder line section as compared with a prior-art nozzle which has a straight (non-slanted) powder line section. But the rate of damage, which can be expressed as a thickness of a damaged portion worn off per unit time, did not substantially change. Thus, as occurred with the straight powder line section-type of submerged nozzle, the gas bubbles moved up directly from the discharge port and floated near the nozzle, making it possible to attain only a relatively slight, advantageous effect attributable directly to the increase in thickness.
Japanese Utility (Laid-Open) Model No. 59-89648 discloses a submerged nozzle provided with a projecting part having a slanting surface which makes an obtuse angle (D) with the longitudinal axis of the nozzle body, in a direction opposite that of the discharge port, bordering an end portion of the discharge port (see present FIG. 4). The submerged nozzle is provided between a tundish or ladle (not shown) and a mold 9. With reference to the submerged nozzle as it is normally used (see FIG. 4), a lower end portion of the submerged nozzle 1 is immersed in a molten steel 10 in the mold 9. A nozzle passage 1a is formed in the nozzle 1 and connected with two or more discharge ports 2 so as to guide a molten metal into the mold 9 in the direction designated by the arrows. A projecting part 4' is formed at an upper end of each discharge port 2 for guiding both the molten metal 5 and the argon gas bubble 3. The projecting part 4' has a slanting surface with a dipping angle to a horizontal line, so that the slanting surface is inclined downwardly. The slanting surface of the projecting part 4' and a slanting surface of the discharge ports constitute a common surface which is inclined downwardly, relative to the direction of fluid flow within the nozzle.
The arrangement shown in FIG. 4 has proved effective, however, only in keeping gas bubbles far from the powder line section, i.e., the gas bubbles ejected from the discharge ports still collide directly against the slanting surface of the projecting part. As a result, damage to the projecting part becomes a more serious problem, to the extent that a reduction in the service life of the projecting part occurs.
Accordingly, in case of the submerged nozzle having a increased thickness at the powder line section, the powder line section must be further improved because it is subject to greater damage in comparison with the other nozzle sections. On the other hand, when a submerged nozzle has a projecting part with a slanting surface that borders an upper end of a discharge port, the projecting part faces the gas bubble flow substantially at a right angle, which produces unavoidable phenomena such as damage by the molten metal to the projecting part. In addition, the flow of gas bubbles is changed into turbulent flow after the collision of the gas bubbles against the projecting part of the nozzle, thereby causing an increase in the agitation effects.